1. Technical Field
This invention relates generally to wireless communication systems, and more particularly, but not exclusively, to a linear in-phase transmitter.
2. Description of the Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channel pair (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel or channel pair. For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver receives RF signals, removes the RF carrier frequency from the RF signals directly or via one or more intermediate frequency stages, and demodulates the signals in accordance with a particular wireless communication standard to recapture the transmitted data. The transmitter converts data into RF signals by modulating the data to RF carrier in accordance with the particular wireless communication standard and directly or in one or more intermediate frequency stages to produce the RF signals.
The IEE 802.11g standard uses Orthogonal Frequency Division Multiplexing (OFDM) modulation that has data information in both the amplitude and phase. In order not to degrade the modulation quality, the transmitter has to be operated 10 dB lower than the 1 dB compression point of the transmitter. That is, the operating output power backs off by 10 dB of its 1 dB compression point. The design target is to send out 5 dBm output power, which means the transmitter has to be linear up to 15 dBm. That is, the transmitter and thus the power amplifier driver (PAD) (the last stage of the transmitter) has to be designed with output 1 dB compression point equal to or more than about 15 dBm. A balun coupled to the PAD combines the differential signal and then sends it to a power amplifier if extra output power is required.
The design of a power amplifier driver (PAD) can be classified as class A, class AB, class B, class C, et cetera when biased at different current levels. In some designs, a programmable gain amplifier (PGA) is coupled to and drives a PAD. Small and big swings in the PGA output cause PAD to present different capacitive loads to the PGA, which causes PGA phase distortion in the time frame. Since information is carried by both amplitude and phase, phase distortion will make the transmitter non-linear.
Accordingly, a system and method are needed in designing the PAD such that it presents a constant capacitive load for its previous stage PGA. The PGA and hence the whole transmitter can then have linear in phase response.